This invention relates to a method and apparatus for processing digital signals and, more particularly, is directed to a method and apparatus for determining the correct state of a control signal contained in transmitted digital signals containing errors, especially digital signals which are recorded and reproduced with a digital video tape recorder.
Recently, digital techniques have been applied to the transmission and recording of video signals. In particular, a rotary head type video tape recorder (VTR) has been used to record pulse code modulated (PCM) video signals on a magnetic tape and, upon playback by the rotary head recorder, the video signals are pulse code demodulated to obtain an analog video signal. In such case, the digital video signals are generally grouped into blocks with each block containing a predetermined number of bits. Upon playback, each block of the reproduced digital video signals is processed as an entity.
However, when a PCM-encoded video signal is recorded and subsequently reproduced, there is the possibility that the reproduced video signals may contain random errors caused by various types of noise, such as head noise, tape noise, and amplifier noise, and may also contain burst errors (signal dropout) resulting from dust, fingerprints, or flaws on the tape surface. It should be appreciated, of course, that such errors may seriously deteriorate the quality of the resulting video picture. In order to minimize this problem, error correction codes have been used in encoding the PCM signals prior to recording on the tape. For example, parity words may be added every predetermined number of blocks of video data and such parity words are then used during the playback process in an error detection operation. By using such error correction codes, erroneous PCM signals may be corrected or compensated so as to avoid the aforementioned deterioration in video playback. It should be appreciated that the more error correction code words that are used, the more accurate is the error detection/correction operation. However, it is also desirable, in achieving such error correction, to reduce the "overhead", or redundancy by keeping the number of error correction bits as small as possible so as to maximize the area of tape that can be used for recording of data.
Further, when the frequency of errors becomes high, so that the number of errors exceeds the error correcting capability of the error correction code, an error concealment operation, rather than an error correction operation, is used. Such operation may be accomplished, for example, by replacing the erroneous video data with video data which are approximately equal thereto. In this regard, a field memory for storing successive fields of video data is provided and an address signal is added to each block of video data for addressing the blocks of video data into the field memory. When the speed of movement of the magnetic tape during playback is faster than that used during recording, the rotary head can be shifted so as to skip over a predetermined number of tracks to reproduce, for example, every other track. During playback at a speed slower than that of recording, the rotary head scans the same track more than once and then jumps over to the next adjacent track. Consequently, the reproduced video data are not of a continuous nature. ln this regard, the address signals of the reproduced video data are used to write the video information into the field memory at predetermined addresses so as to obtain a picture having continuity.
When the aforementioned error concealment operation is utilized with a digital color video signal, the phase of the color sub-carrier may be inverted at the connection point between the original erroneous video data and the substituted video data. More particularly, in the case of an NTSC system, consecutive frames are said to alternate between "odd" and "even" frames; that is, the phase of the color subcarrier between corresponding portions of successive frames differs by .pi./2. Similarly in successive fields the color subcarrier phase also differs by .pi./2, and in consecutive line intervals, the color subcarrier phase also differs by that amount. It should therefore be appreciated that when video data (or field or line) from one frame are substituted for corresponding video information of a successive frame (or field or line), the phase of the color sub-carrier of the substituted video information must be inverted to maintain continuous phase relation of the color sub-carrier. This is explained more fully in U.S. patent application Ser. No. 194,830, filed Oct. 7, 1980, having a common assignee herewith. In this regard, it has been proposed to add an identification signal to the video data for indicating the frame, field, and line to which the video information belongs, or at least to identify whether the frame, field, or line is even or odd. However, if an error results in the identification signal, such phase inversion cannot reliably be performed.
Moreover, to correct more accurately any error caused by drop-out, it has been proposed to add still another error correction code to the video data signal for use in detecting and correcting any error occurring in the address signal in the identification signal in each block of video data. However, such a code having a high capability of error detection and correction tends to make the recorded digital video signal overly redundant, and at the same time requires a high degree of circuit complexity and sophistication, both for the recording and for the playback operations.